An image may be encoded into image data, which may be transmitted, stored, processed, or otherwise manipulated electronically. The image may be decoded and converted to hard copy by sending the image data to a printer. Laser thermal printers, with their high resolution capability, provide a popular mode for producing hard copy images from digital image data. Laser thermal printers may be used with a variety of imaging media or “receptors,” including many kinds of film and paper. In a typical laser imager, a receptor is placed very close to a color-coated substrate or “donor” sheet, and a plurality of laser beams are directed at the donor. Each laser may emit an infrared beam, and the colored coating, which may contain a colorant such as an infrared-sensitive dye, heats when exposed to a beam. The resulting thermal energy induced by the lasers triggers the imaging process, causing colorant to transfer from the donor to the receptor.
The lasers are typically arranged in a linear array, with each laser in the array individually modulated by image data. The array may include any number of lasers, although an array of sixteen lasers is typical. Semiconductor or “diode” lasers are commonly used in an array for reasons of cost, convenience and reliability. The lasers may, for example, emit infrared beams with wavelengths of 830 nm. The width of the array, which is a function of the spacing of the lasers, is usually adjustable.
The image data that modulate the lasers represent the shape, size, color and density of the image. Image data are routinely stored electronically, and are provided to the array in the form of a plurality of signals, typically one signal for each laser. Although the lasers in the array strike only a small portion of the donor and receptor at any one time, the array can print large regions by scanning across the donor and receptor. As the array scans the donor and receptor, each laser in the array emits a be in response to an image signal. In most cases, the laser array may make several successive parallel or helical passes to generate the complete image. Each pass of the array prints a strip or “swath” on the receptor. To avoid the appearance of white lines in the receptor, i.e., unprinted spaces between swaths, successive swaths may abut or overlap preceding swaths.
When a beam sufficiently heats the donor, a spot of colorant is transferred from the donor to the receptor. By modulating the duration for which a laser beam strikes an area on the donor, modulating a laser's intensity and/or modulating the size of the beam, spots of colorant of different sizes may be formed, and thereby colors may appear darker or fainter in color. Often a region of the receptor is intended to receive no colorant from the donor, and consequently a laser emits no beam when scanning that region.
The receptor may be scanned multiple times using donors of different colorants, creating a multicolor image by the superposition of multiple monochromatic images. By repeating scans with donors coated with cyan, yellow, magenta and black, for example, a multicolor image may be formed on the receptor. For high fidelity printing systems, additional colors such as green and orange may be provided.